Curing polyepoxide resin binders in high density syntactic foam forming compositions

ABSTRACT

Disclosure is made of a method of curing the polyepoxide resin binder component of polyepoxide based syntactic foams of high density. Ketones are employed to reversably block primary amine groups on polyamine curing agents during the initial curing. As the cure progresses, the block is removed to complete the cure. The result is a moderation of peak exotherms generally associated with the cure, inhibiting degradation of the product foam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to syntactic foams and more particularly relatesto curing syntactic foam made with polyepoxide resin binders.

2. Brief Description of the Prior Art

The literature is replete with descriptions of prior art syntactic foamsmade by curing mixtures of microspheres with curable polyepoxide resinbinders; see for example the description given in the U.S. Pat. No.3,849,350.

The prior art techniques of curing the polyepoxide resin binders insyntactic foam forming compositions have been, in general, satisfactorywhen the article to be molded is relatively low in density i.e.; havinga density below about 35 lbs./cubic foot. Higher density mouldings arenot always satisfactory. The difficulty arises because the curingprocess is a chemical reaction, exothermic in nature. The general heatof reaction is desirable to sustain the curing reaction to completion.However, the syntactic foam being formed is a thermal insulator and asthe exotherm continues, thermal energy is trapped within the body of themoulding. In high density mouldings the heat build-up on the interior ofthe moulding can exceed temperatures which are degradative to the foamcomposition, i.e.; above about 400° F. When this happens, the resinbinder of the moulding will degrade, split, char, become brittle,discolor and weaken.

By the method of the present invention, relatively dense mouldings maybe made from syntactic foam forming compositions having a curablepolyepoxide resin binder ingredient without exceeding internaltemperature limits which would be degradative of the cured polyepoxideresin binder. The mouldings are particularly useful as buoyancy devicesin marine applications.

SUMMARY OF THE INVENTION

The invention comprises a method of curing the polyepoxide resin bindercomponent of a syntactic foam forming composition, which comprises inadmixture a curable polyepoxide resin matrix, and a filler whichcomprises a plurality of hollow microspheres, said foam having a densityin excess of 35 lbs./ft³, comprising the steps of; curing thepolyepoxide component with a curing agent which is the reaction productof (1) an organic polyepoxide curing agent possessing a primary aminegroup and (2) a ketone of the formula: ##STR1## wherein R₁ and R₂ areeach independently selected from the group consisting of monovalentorganic radicals.

The term "monovalent organic radical" as used throughout thespecification and claims means an organic molecule including hydrogenatoms, one of which has been removed. Representative of monovalentorganic radicals are hydrocarbyl. The term "hydrocarbyl" as used hereinmeans the monovalent moiety obtained by removal of a hydrogen atom froma parent hydrocarbon, which latter, for example, contains 1 to 12 carbonatoms. Illustrative of such moieties are alkyl of 1 to 12 carbon atoms,inclusive, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, nonyl, decyl, undecyl, dodecyl and isomeric and unsaturated formsthereof; cycloalkyl of 3 to 8 carbon atoms, inclusive, such ascyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl and the like; aryl of 6 to 12 carbon atoms, inclusive, suchas phenyl, tolyl, xylyl, napthyl, biphenyl and the like, aralkyl of 7 to12 carbon atoms, inclusive, such as benzyl, phenethyl, phenpropyl,phenbutyl, phenpentyl, phenhexyl and the like.

The term "hollow microsphere" as used herein means a microscopic,hollow, spherical capsule such as is described in the U.S. Pat. No.4,303,730.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The syntactic foams prepared by the method of the invention contain as acomponent, fillers which are hollow microspheres. Hollow microspheresare well-known filler materials as is the method of their preparation.For example, microspheres made of borosilicate glass in diameters offrom about 20 to 10,000 microns are commercially available. Theirpreparation is well-known; see for example U.S. Pat. Nos. 4,303,730 and4,303,732. Hollow microspheres of synthetic polymeric resin materialssuch as polystyrene are also known and have been used in the making ofsyntactic foams.

The syntactic foams prepared by the method of the invention may alsocontain as filler materials, hollow macrospheres such as is described inthe U.S. Pat. No. 3,622,437. These macrospheres are generally formed ofthermoplastic, synthetic polymeric resins in sizes having diameters offrom about 1/2 to 4 inches.

The filler components of the syntactic foams are held together in apolyepoxide resin matrix or binder component. In their preparation, acurable foam forming composition is made by admixture of a curablepolyepoxide resin with from 20 to 80 percent by volume of fillers. Theresin component is then cured, generally with an organic amine curingagent to obtain the syntactic foam.

The method of the invention may be employed to cure any polyepoxidesyntactic foam forming binder resin. Thus, the polyepoxide may bealiphatic, cycloaliphatic, aromatic or heterocyclic in nature.Representative of such polyepoxide are:

(1) Diglycidyl ethers of aliphatic diols having 2 to 6 carbon atoms,inclusive, such as ethane-diol, propanediol, butanediol, pentanediol,hexanediol and isomeric forms thereof;

(2) The glycidyl ethers of polyhydric mononuclear and fused ring phenolssuch as resorcinol, hydroquinone, pyrocatechol, saligenin,phloroglucinol, 1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene,1,7-dihydroxynaphthalene and the like;

(3) The glycidyl ethers of non-fused polynuclear phenols represented bythe general formula: ##STR2## wherein R₃ represents from 0 to 4substituents selected from the class consisting of halogen such aschlorine and bromine, and lower-alkyl, A is a bridging group selectedfrom the class consisting of ##STR3## and a single covalent bond,wherein R₄ and R₅ each represent a moiety selected from the classconsisting of hydrogen, lower-alkyl, lower-cycloalkyl and aryl.Illustrative of such compounds are the bis(glycidyl ethers) of:

4,4'-dihydroxydiphenylsulfone,

4,4'-dihydroxybiphenyl,

4,4'-dihydroxybenzophenone,

di(4-hydroxyphenyl)methane (bisphenol F),

2,2-di(4-hydroxyphenyl)butane (bisphenol B),

2,2-di(4-hydroxyphenyl)propane (bisphenol A),

1,1-di(4-hydroxyphenyl)propane,

3,3-di(3-hydroxyphenyl)pentane,

2-(3-hydroxyphenyl)-2-(4'-hydroxyphenyl)butane,

1-phenyl-1-(2-hydroxyphenyl)-1-(3'-hydroxyphenyl)butane,

1-phenyl-1-(2-hydroxyphenyl)-1-(3'-hydroxyphenyl)propane,

1-phenyl-1,1-di(4-hydroxyphenyl)butane,

1-phenyl-1,1-di(4-hydroxyphenyl)pentane,

1-tolyl-1,1-di(4-hydroxyphenyl)ethane,

bis(3-bromo-4-hydroxyphenyl)methane,

2,2-bis(3-bromo-4-hydroxyphenyl)propane,

bis(3-bromo-4-hydroxyphenyl)diphenylmethane,

1,1-bis(3-bromo-4-hydroxyphenyl)-1-(2,5-dibromophenyl)ethane,

bis(3,5-bromo-4-hydroxyphenyl)methane,

2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane,

bis(3,5-dibromo-4-hydroxyphenyl)diphenylmethane,

1,1-bis(3,5-dibromo-4-hydroxyphenyl)-1-(2,5-dibromophenyl) ethane,

bis(3-bromo-4-hydroxyphenyl)sulfone and

bis(3,5-dibromo-4-hydroxyphenyl)sulfone; and

(4) The glycidyl ethers of novolak resins. The novolak resins are theproducts obtained by acid condensation of phenol, or a substitutedphenol, with formaldehyde and are conventionally represented by thegeneral formula: ##STR4## wherein n has an average value of from about 8to 12 and R₆ represents from 0 to 4 substituents selected from halogenand lower alkyl groups. It is to be understood that the above formula ishighly idealized and is an approximation only; see, for example,Carswell, "phenoplasts," pp. 29-35, Interscience, New York, 1947. A widerange of novolak resins of differing molecular weights is availablecommercially, all of which are represented approximately by the aboveformula. Since the class of novolak resins is so well recognized in theart, the epoxides derived therefrom by conversion of the novolaks totheir glycidyl ethers (by conventional procedures, e.g., reaction withepichlorohydrin) may be referred to as "novolak resin glycidyl ethers."

Epoxide compounds characterized by having two terminal epoxy groupsseparated by the residue of a polyoxyalkylene glycol having a molecularweight of from about 100 to about 700 are well-known. They areillustrated by diepoxides of the formula: ##STR5## wherein Y and Y' areeach selected from the group consisting of hydrogen and methyl and n isan integer of from 1 to 10, inclusive. Representative of such diepoxidesare those prepared by reaction of two moles of epichlorohydrin with onemole of a polyoxyalkylene glycol having a molecular weight of from about100 to about 700. Polyoxyalkylene glycols are well-known compounds asillustrated by polyoxyethylene glycol and polyoxypropylene glycol.

The curing of a polyepoxide occurs primarily when there is across-linking between the epoxide groups of two separate polyepoxidemolecules resulting in an infusible product resin. In the method of thisinvention, polyamines are employed as cross-linkers, the activehydrogens serving to open the oxirane rings on the polyepoxide andpermit insertion of the nitrogen atom from the polyamine onto a carbonatom formerly part of the oxirane ring.

Polyamine agents for curing polyepoxides are well-known as is the methodof their preparation. Representative of such agents are those of theformula: ##STR6## wherein R₇ represents hydrocarbylene and n is aninteger of from 1-5.

The term "hydrocarbylene" as used herein means the divalent moietyobtained upon removal of a hydrogen atom from a hydrocarbyl radical aspreviously defined.

Representative of polyamine curing agents of the formula (V) given aboveare ethylene diamine, diethylene triamine, diethyleneaminopropylamine,m-phenylenediamine, p-phenylenediamine, methylenediamine, triethylenetetramine, tetraethylene pentamine, and the like.

In the method of the invention, the conventional and known polyaminecuring agents such as those of the formula (V) given above are used, buttheir cross-linking ability is delayed or inhibited by reversablyblocking all or some primary amine groups. During the curing period, theprimary amine groups are unblocked. Normal cross-linking progresses overa longer than normal time period. This moderates the exotherm and thebuild-up of heat by allowing heat dissipation to occur at a rate greaterthan the thermal accumulation.

Although the inventors are not to be bound by any theory of operationfor the method of their invention, a theoretical explanation of themoderated exotherm experienced may be postulated. It is believed thatthe ketone of formula (I) given above reacts with primary amine groupson the curing agent according to the schematic formulae: ##STR7##wherein R₁ and R₂ are as previously defined. The addition compound offormula (VI) is unstable and eliminates a molecule of water to form theimine of formula: ##STR8## thereby removing all active hydrogen atomsfrom the amine moiety. The ketone blocked polyamine therefore has fewerreactive sites for cross-linking the polyepoxide and there is aconsequent lower exotherm. However, the C═N bond is hydrolyticallyunstable and as the cross-linking reaction continues, in the presence ofthe water, hydrolysis occurs to reverse the above described reactionbetween ketone and primary amine groups. As the regenerated primaryamine groups appear, they become available to enter into reaction withunreacted epoxy groups, thereby completing the cure of the polyepoxideresin albeit at a time substantially after initial reaction of thepolyepoxide resin and the curing agent. In this way, the period of theexotherm is drawn out to moderate high temperature through accumulationor retention of heat values within the body of the syntactic foamcomposition undergoing cure.

Preferred polyamine curing agents employed in the method of theinvention are polyaminoamides and polyaminoimidazolines.

The polyaminoamides are carboxylic acid amides or polyamides containingone or more free amine groups which are directly attached to nitrogen(definition given in U.S. Pat. No. 2,760,944). Preferred for use in thepresent invention are polyaminoamides of the general formula: ##STR9##polyaminoimidazolines derived therefrom and adducts thereof withpolyepoxides, wherein R₈ is the divalent residue obtained upon removalof a carboxyl group from an unsaturated fatty acid or when n is at least1, two carboxyl groups a polymerized unsaturated acid; R₇ representshydrocarbylene as previously defined (preferably alkylene of 2 to 5carbon atoms, inclusive, eg; ethylene, propylene, butylene orpentylene); m is an integer of from 1 to 4 and n is an integer of 0 to4.

The polyaminoamides of the formula (VIII) given above are generallywell-known as is the method of their preparation; see for example the"Handbook of Epoxy Resins", Henry Lee and Kris Neville, Chapter 10 p. 5[McGraw-Hill Book Company, (1967)] for the preparation and structure ofsimple polyaminoamides and U.S. Pat. No. 3,002,941 for the preparationof more complex polyaminoamides.

In general, the preferred polyaminoamides of formula (VIII) may beprepared by the condensation of unsaturated fatty acids or polymerizedunsaturated acids with polyhydrocarbylene polyamines such as those ofthe formula (V) given above.

The unsaturated fatty acids are also well-known and include for examplepalmetic acid, stearic acid, elaidic acid, linoleic acid, linolenicacid, dehydrated castor oil fatty acid, elaostearic acid, tall oil fattyacids and the like.

Polymerized fatty acids are advantageously employed in the preparationof polyaminoamides of the formula (VIII) or the correspondingpolyaminoimidazolines.

The polymeric fat acids are a mixture of dimeric and trimeric fattyacids resulting from the polymerization of drying or semi-drying oils orfrom the polymerization of the free acids or simple aliphatic alcoholesters of the acids of such oils as soybean, linseed, tung, cottonseed,corn, tall, sunflower, safflower and dehydrated castor oils. These maybe polymerized by a simple thermal polymerization in which case thefatty acids of sufficient double bond functionality combine to provide amixture of dibasic and higher polymeric acids. The acids withinsufficient functionality to react remain as monomers and may beremoved by distillation. Inasmuch as some monomeric fatty acid may bedesirable to control molecular weight as was indicated above, the amountof monomer left in the polymeric fat acids may be regulated for thispurpose. In place of thermal polymerization for the production of thepolymeric fat acids a catalytic method of polymerization such as, forexample, the polymerization of mono or poly olefinic acids in thepresence of such catalysts as ditertiary butyl peroxide may be employed.The resultant polymeric fat acids may retain residual unsaturation ormay be saturated either as a result of the polymerization reaction orthrough hydrogenation.

Polyaminoimidazolines derived from the polyaminoamides are alsowell-known and may be prepared from polyaminoamides according to thescheme: ##STR10## by simply heating the polyaminoamide. If heating isstopped before there is a complete conversion to thepolyaminoimidazoline, a compound is obtained having both amide andimidazoline groups or moieties. Strictly speaking, thepolyaminoimidazolines employed in the invention need not be "derived"from polyaminoamides since it is well-known that they may also beprepared from the appropriate carboxylic acids directly according to thegeneral scheme: ##STR11## as described in The Handbook of Epoxy resins,supra.

Adducts of the polyaminoamides and the polyaminoimidazolines describedabove derived by reaction with polyepoxides (see U.S. Pat. No.3,474,056) may also be used as curatives in the method of the invention.

Advantageously the polyaminoamides and polyaminoimidazolines used in theinvention have amine numbers of from 20 to 500, preferably 360 to 390.The "amine number" is defined as the number of milligrams of potassiumhydroxide equivalent to the free amine groups of the resin.

The imines formed by the reaction of the ketones of formula (I) with thepreferred polyaminoamides of formula (VIII) may be represented by thegeneral formula: ##STR12## wherein R₁, R₂, R₇, R₈, m and n are asdefined above, x is an integer of 0 to 3 and y is an integer of 0 to 3;and A is selected from the group consisting of the radical ##STR13## andhydrogen. The imines of formula (IX) and the relatedpolyaminoimidazolines are prepared by mixing the ketones (I) and thepolyaminoamides (VIII) or the corresponding polyaminoimidazolinestogether at ambient (room) temperatures in stoichiometric proportions,i.e.; a mole of ketone (I) for each primary amine group to be convertedto an imine group.

Ketones of the formula (I) are generally well-known compounds as is themethod of their preparation. Representative of the ketones (I) areacetone, methylethyl ketone, diethylketone, methylpropyl ketone,dipropyl ketone, methylisobutyl ketone and the like. Preferred in themethod of the invention is acetone.

The solid syntactic foam forming composition is prepared by simpleadmixture of the composition ingredients employing conventional mixingapparatus and techniques. Preferably the curing agent and the ketone offormula (I) are premixed and reacted to obtain the modified curingagent.

The proportions of ingredients mixed together to obtain the syntacticfoam forming composition are, in general, conventional. The proportionof microspheres or other fillers may constitute from 50 to 75 percent byvolume of the composition. The proportion of curing agent is thatrequired to cure the polyepoxide resin ingredient.

Additional ingredients may be added to the foam forming compositions asis conventional in the art. Representative of such additionalingredients are inert solvents for the epoxide reactant, inert fillersand reinforcing materials such as textile fibers and the like, flameretardants such as potassium and antimony salts and the like.

The following examples describe the manner and process of making andusing the invention and set forth the best mode contemplated by theinventors for carrying out the invention but are not to be construed aslimiting. All specified test results, where given, were according to thefollowing procedures.

Compressive strength and compressive modulus tests were performedaccording to the ASTM procedure D-695.

Flexural strength tests were performed on cylindrical specimens with adiameter of one-half inch with a three inch support span, and using theequation. ##EQU1## wherein P is the applied load at failure, L is thespan and D is the specimen diameter.

Water absorption tests were carried out on discs with a diameter of 23/4inches, and a thickness of 3/4 inch. The test procedure consists of 550ps hydrostatic pressure for up to 912 hours and the test results arereported as daily measurement.

EXAMPLE 1

An appropriate vessel was charged with 500 gms of a polyaminoamidehaving an amine number of 375 (Versamid 140, Henkel Company). To thecharge there was added with stirring 21.8 gms. of acetone. A mildexotherm was noted. After the exotherm ceases, the mixture was ready foruse as a polyepoxide curing agent.

EXAMPLE 2

The procedure of Example 1, supra., was repeated except that in place ofthe 21.8 gms. of acetone was 43.6 gms added to the polyaminoamide.

EXAMPLE 3

As appropriate vessel was charged with 1500 gms of a diepoxide which isthe bis(glycidyl ether) of 2,2-di-(4-hydroxyphenyl) propane (bisphenolA) having an average molecular weight of 370 and an epoxy equivalentweight of 185 (Epotuf 37127, Reichhold Chemicals, Inc.). To the chargethere was added with stirring 430 gms of hollow glass microsphereshaving a density of 0.15 gms/cc (Glass Bubbles, type (-15/250, 3MCompany, St. Paul, Minn.; 90 percent by volume having a diameter sizebetween 20 and 130 microns) and the curing agent prepared in Example 1,supra. The resulting solid foam forming mixture was cast into a numberof 12 inch diameter waxed cardboard tubes of varying height (10 to 16inches) and thermocouples are positioned at the center of the castings.The tubes were placed in an oven maintained at ambient temperature.After a period of about 16 hours the oven was adjusted to a temperatureof about 180° F. and the castings were allowed to post-cure for a periodof 10 hours. Representative portions of the cured foam were thensubjected to physical testing. The test results and the thermocouplemeasurements are given in Table I, below.

The above procedure was repeated, except in place of the curing agent asused therein, 500 gms. of Versamid 140, supra., was used. The resultingsolid foam serves as a control, the physical properties and thermocouplemeasurements being given in the Table I, below.

Differential scanning calorimeter tests showed that the above foams didnot contain unreacted epoxy groups.

EXAMPLE 4

The procedure of Example 3, supra., was repeated except that the curingagent as used therein was replaced with the curing agent prepared inExample 2, supra. Differential scanning calorimeter tests did not revealany unreacted epoxy groups.

                  TABLE I                                                         ______________________________________                                                     EXOTHERM                                                                      TEMPERATURE                                                                   (°F.)/CARDBOARD                                                                         HEIGHT                                                       TUBE             (INCHES)                                        Sample I.D.  10        12         16                                          ______________________________________                                        Control      403       403        423                                         (as described in                                                              Example 30)                                                                   Example 3 Foam                                                                             372       381        390                                         (curing agent                                                                 prepared in Example                                                           1, supra)                                                                     Example 4 Foam                                                                             330       290        376                                         (curing agent                                                                 prepared in Example                                                           2, supra)                                                                     ______________________________________                                                     38 Day    Compressive                                                                              Compressive                                              Hydrostatic                                                                             Strength   Modulus                                                  Absorption                                                                              (PSI)      (PSI)                                       ______________________________________                                        Control      1.07%     3,401      1.62 × 10.sup.5                       Example 3 Foam                                                                             0.91%     3,777      1.74 × 10.sup.5                       Example 4 Foam                                                                             1.10%     3,315      1.36 × 10.sup.5                       ______________________________________                                    

What is claimed:
 1. A method of curing the polyepoxide resin bindercomponent of a syntactic foam forming composition, which comprises inadmixture a curable polyepoxide resin matrix, and a filler whichcomprises a plurality of hollow microspheres, said foam having a densityin excess of 35 lbs./ft³, comprising the steps of; curing thepolyepoxide component with a curing agent which is the reaction productconsisting essentially of (1) an organic polyepoxide curing agentpossessing a primary amine group and (2) a mode of a ketone of theformula: ##STR14## wherein R₁ and R₂ are each independently selectedfrom the group consisting of monovalent organic radicals for each ofsaid amine groups, said reaction product having been premixed andreacted before addition to the curable polyepoxide resin bindercomponent, whereby said curing occurs.
 2. The method of claim 1 whereinsaid ketone is acetone.
 3. The method of claim 1 wherein said reactionproduct has the formula: ##STR15## wherein R₁ and R₂ each representhydrocarbyl; R₇ represents hydrocarbylene; A is selected from groupconsisting of the radical ##STR16## and hydrogen; x is an integer offrom 0 to 3; y is an integer of 0 to 3; and R₈ represents the residue ofa polymerized fatty acid following removal of two carboxyl groups.